Gold nanoparticles are widely used in different fields. They are currently under extensive investigation with regard to their linear and nonlinear optical properties. So far, nanoparticles with centrosymmetrical shapes, such as nanospheres or nanorods, have received the main attention. In this work, the properties of gold nanoprisms exhibiting a highly noncentrosymmetrical threefold symmetry are investigated with hyper-Rayleigh scattering (HRS). Aqueous solutions of gold nanospheres and nanoprisms with different sizes were synthesized. The first hyperpolarizability magnitudes of both the nanospheres and nanoprisms were determined separately with the assumption of a surface origin of the nonlinearity. Using polarization-resolved HRS, retardation is shown to be largely underdeveloped for the nanoprisms as opposed to the nanospheres. The nanoprism shape noncentrosymmetry has therefore a leading role in the HRS response although it is also shown that surface defects induce deviations from the ideal threefold symmetry.

Nanoparticles made of a substance derived from wood, lignin, are gaining interest as a potential material for use in nanotechnology, including such biological applications as photodynamic antimicrobial chemotherapy (PACT) and photodynamic therapy (PDT), which involve the need for the nanoparticles to contain photosensitizers that absorb and emit in certain wavelength ranges. Among the possible approaches there is one relying on utilizing two-photon absorption in the near infrared region. In this work, we present the linear and non-linear photophysical properties of acetylated lignin nanoparticles which have been loaded with porphyrin, in particular, the femtosecond laser-induced emission which was measured for a single nanoparticle. We demonstrate that these nanoparticles can be excited in the so-called first (NIR-I) and second (NIR-II) optical window in the near-infrared (650–950 nm and 1000–1350 nm, respectively). Because their emission also occurs in the NIR, this presents an advantage for the monitoring of the use of these porphyrin-particle hybrids within PDT and PACT applications.